1) Field of the Invention
The present invention relates to composite fasteners for fastening structural members, i.e., fasteners formed at least partially of a composite material, such as hybrid fasteners formed of a combination of composite and metallic materials, and/or blind composite fasteners.
2) Description of Related Art
High-strength composite materials are increasingly being used in the manufacture of various types of structural members due to improved physical properties and economic availability of the materials. Along with this increased use, the composite materials are more commonly being used in conjunction with metallic components in situations that require structural integrity and fatigue resistance. For example, structural members for aerospace applications that are formed of high-strength composites such as carbon/epoxy, polyaramid/epoxy, or glass/epoxy composites are mechanically fastened using high-strength metallic aerospace fasteners. Such fasteners are typically formed of metallic materials such as titanium and stainless steels that are different from the composite materials of the structural members. Therefore, the fasteners often have physical characteristics that are dissimilar to or incompatible with the composite materials of the structural members being fastened. In some cases, the use of dissimilar materials can result in delamination of the composite structural members caused by the fastener under installation and operational loads, inadequate sealing surrounding the fastener and therefore leakage of fluids through the joined components, inadequate electrical continuity between the composite components and arcing across the gaps surrounding the fasteners, galvanic corrosion between the fasteners and components at the composite joint, increased weight of the joined components, and/or nonuniform coefficients of thermal expansion between the fastener material and the surrounding composite material.
Still, metallic fasteners can also be provided with an organic corrosion-inhibiting coating. The metallic fasteners provide adequate strength, and the organic coating improves the compatibility of the metallic fastener with the joined composite materials. However, the dissimilarity of the metallic fastener material properties and the composite materials joined in the assembly can still result in delamination, inadequate electrical continuity, galvanic corrosion, and nonuniform expansion between the materials.
Some fasteners formed of composite materials such as PEEK™ can be subject to the same problems, as noted above. Further, current fasteners formed of composite materials are generally not adapted for blind installation. For example, the installation of two-piece composite fasteners such as threaded pins or bolts generally requires the threaded pin or bolt to be inserted from a first side of the structural members being joined with access for installation of a nut at the opposite side. Similarly, one-piece composite rivets generally must be inserted from a first side of the structure to be assembled and then upset at the opposite side. Such fasteners, either one-piece or two-piece, generally cannot be used for fastening applications in which one side of the structural members is inaccessible. Further, even if both sides are accessible, access to both sides of the fastener can require additional tooling and/or processing steps.
Thus, a continued need exists for improved fasteners that can be used with either composite or metallic and composite structural members for aerospace and non-aerospace applications. Preferably, the fasteners should be adaptable for fastening multiple composite structural members or a combination of composite or metallic structural members while reducing or eliminating the occurrence of problems relating to delamination, electrical continuity, galvanic corrosion, and nonuniform expansion that are associated with metallic fasteners and conventional composite fasteners. Further, the fasteners should be adaptable for blind installation, that is, an installation having no or limited access from one side of the structure to be assembled.